Advanced Ni-base superalloys are currently isothermally forged 60 at relatively slow strain rates and temperatures below their .gamma.' solvus temperatures. Forging 60 is typically followed by a combination of subsolvus and supersolvus annealing 70 as illustrated in FIG. 1, and may be combined with controlled cooling 75. This method utilizes the superplastic deformation of Ni-base superalloys and tends to minimize forging loads and die stresses, and avoids fracturing the items being formed during forging operations. The superplastic deformation is of particular benefit in that it permits more complex shapes to be forged, it also permits the retained metallurgical strain in the forging at the conclusion of the forming operations to be minimized. However, this method can have substantial limitations with respect to forming substantially uniform fine grain size articles. While the method tends to produce relatively fine-grain as-forged microstructures having an average grain size on the order of about 7 .mu.m, subsequent supersolvus annealing causes the grain size to increase to about 20-30 .mu.m. Also, unless the forging process is carefully controlled so as to avoid imparting retained strain into the forged articles, this method can produce articles that are subject to the problem of critical grain growth, wherein the retained strain energy in the article can cause limited nucleation and substantial growth (in regions containing the retained strain) of very large grains upon subsequent supersolvus annealing. Critical grain growth can cause the formation of grains as large as 300-3000 .mu.M.
Also, in advanced applications such as turbine disks, it may be desirable to have location specific properties within a given article, such as a finer grain size in the bore for enhanced low temperature strength and low cycle fatigue (LCF) resistance; coupled with a larger grain size in the rim for crack propagation resistance and high temperature creep resistance. The related art forging method described above also has not been shown to be suitable for producing such location specific properties.
Therefore, new methods of forging are desirable that retain the benefits of isothermal forging, such as the use of superplastic deformation to form more complex shapes, and yet also produce forged articles that avoid critical grain growth. It is also desirable that such new methods that enable the development of location specific alloy properties.